Pylon device could double capacity of UK power networks

UK-developed technology that can be fitted to existing electricity pylons could dramatically increase the capacity of transmission networks without the need for expensive rebuild programmes.

Jointly developed by engineers at Manchester University of and EPL Composite Solutions, and manufactured by Manchester spin-out firm Arago, the device, a form of insulated cross-arm, addresses some of the fundamental design limitations of the traditional pylon.

Prof Simon Rowland, who has led the work at Manchester’s School of Electrical and Electronic Engineering, and is a director of Arago, told The Engineer that the device could enable operators to increase the power carrying capability of their powerlines by up to 2.5 times.

‘The existing transmission network is really not big enough any more,’ explained Rowland. ‘All the transmission operators in the developed world are trying to get more and more power down their existing lines’

Rowland added that although operators can achieve small improvements by using different conductors or adjusting cable tension, the only way to significantly improve transmission capability has been installing completely new pylons, an expensive process that’s often fraught with planning implications.

A standard pylon features six metal latticework cross-arms from which the conductors hang, separated from the cross-arms by the distinctive-looking vertical string of insulating discs

Arago’s device combines the cross-arm and insulator in a single structure made from the insulation materials: in this case pultruded glass composite, and silicon rubber.

This has a number of benefits, explained Rowland. Firstly, because there is no hanging insulator, the conductor can be placed higher up the tower, allowing operators to put more current through the cable as it can sag more when it gets hot.

Dispensing with the hanging insulator also means that operators don’t need to allow so much clearance to prevent high-winds from blowing the conductors into the side of the tower. ‘Suddenly we have more clearance to play with and can increase the voltage of the line,’ said Rowland. ‘In principle you can take a 132kv line and turn it into a 275kv line or take a 275kv line and turn into a 400kv line. Without massive planning problems you can increase the transmission capability of your line.’

Rowland claimed that the device could also make it easier to connect forms of renewable power generation to the grid. ‘If you have a wind farm on what used to be a quiet part of the grid suddenly you’ve got to find a way of reliably getting more power down that piece of the grid – and it wasn’t really designed to cope with offshore, or even on-shore wind farms.’

Arago is now planning to install 12 Cross-arms with energy companies Scottish Hydro Electric Transmission (SHETL) and National Grid (NG), and is confident that the wider energy industry will soon catch-on to its benefits. ‘Quite when it will become a routine standard piece of equipment is not clear,’ said Rowland, ‘but we have every confidence that it will become that way.’

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The use of plastic composites for overhead line insulation is not a new idea. The team of research engineers under the late Drs J S Forrest FRS and P R Howard carried out extensive investigations at the High Voltage Laboratories of C.E.R.L. in the 1960’s and 70’s into the use of plastics. While they showed promise under laboratory conditions, when subjected to the real world environment of rain, ice, snow and all manner of pollutants including smoke, smog, sand, dust and salt as exemplified by the conditions at their Brighton Insulator Test Station on the South Coast, they failed to provide the sort of long term reliability that transmission engineers have come to expect from good quality glazed porcelain or glass. Both of these conventional and well tried materials have the ability to withstand intense electrical surface discharges without deteriorating whereas the plastic component of composites erodes leaving a porous surface made up of only composite filler and subject to total insulation failure. I also note from the photograph of the cross arm that the number of parallel paths from the conductor to earth is increased by a factor of at least two which has consequences for flash-over probability and fixed transmission losses. This is an issue that the designers need to address. However I look forward to reading about the progress made with trials of this new idea.

In response to the comment by John Steel I would note that composite insulators are now widely deployed at the highest voltage levels. The problems he described were encountered in the 60’s and 70’s but there is now considerable service history of composite insulators on EHV networks including the National Grid.

Re John’s other points. I think the novelty here is more about converting existing steel cross arms to insulating cross arms to raise the line voltage as composite insulators are now established technology. Regarding the flash over comment, this occurs through the air, by definition bypassing the insulators, and hence the number of insulators shouldn’t effect reliability.